1. Field of the invention
It is well known and accepted that to achieve maximum efficiency in a nut and bolt fastening it is necessary to tighten the nut and bolt assembly to such an extent that the tension generated in the bolt shank reaches and preferably exceeds a certain predetermined minimum. This is particularly important in certain applications such as high strength bolting connections and also in nut and bolt fastenings used in the aircraft and automobile fields where it is essential to ensure that nut and bolt assemblies are correctly tightened and that there shall be no slackening of the connection under working conditions where the nut and bolt assembly is subjected to vibrations and other fluctuating stresses.
2. DESCRIPTION OF THE PRIOR ART
There have been proposals for certain forms of nuts and washers which are termed "load indicating nuts or washers" in which the nut or washer is so formed that when tightened in a nut and bolt assembly and when the tension in the shank of the bolt reaches or exceeds a predetermined minimum value a portion or portions of the nut or washer become plastically deformed, the deformation being such as to give a detectable indication in such a way that it can be known that the desired minimum tension in the bolt shank has been reached or exceeded.
The present invention is concerned with the type of load indicating nut or washer wherein the aforesaid deformation results in a movement of part of the nut or washer towards the abutment surface against which a nut is being tightened. In the case of a load indicating nut the abutment surface could be the surface of the work or, if there is a normal load bearing washer under the nut then the abutment surface could be the outer surface of the washer. Likewise in the case of a load indicating washer being used with a conventional nut the abutment surface could again be the surface of the work or, if there is a normal load bearing washer being used under the load indicating washer than the abutment surface could be the outer surface of such load bearing washer.
Hereinafter for convenience of terminology the expression "load indicating element" is to be taken as referring to either a load indicating nut as such or a load indicating washer as such except where the context otherwise specifies. Thus the expression "load indicating element of the type specified" means a load indicating nut or load indicating washer of the type above defined.
An example of a load indicating nut of this type is disclosed in out prior British Specification 1.370.792 in which the part of the nut which undergoes plastic deformation is an annular flange at the end of the nut which engages the abutment and when such plastic deformation takes place there occurs a reduction in the overall axial height of the nut measured between a datum point on the nut body and the abutment surface against which the nut is being tightened. However, with this type of load indicating element there is a requirement to check that the desired amount of deformation has taken place and this involves a separate inspection operation using a gauge such as the "clock" gauge disclosed in the aforesaid patent specification. Other proposals for this type of load indicating element also generally require subsequent inspection and checking with some form of gauge.